P0138
O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)P0138 is a generic OBD-II powertrain diagnostic trouble code: O2 Sensor Circuit High Voltage (Bank 1 Sensor 2). It is logged by the engine control unit when the o2/lambda monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0138 means
P0138 is set when the Engine Control Module (ECM) detects that the upstream-of-catalytic-converter oxygen sensor signal on Bank 1, Sensor 2 is producing a voltage that remains persistently above the accepted threshold — typically above 0.9 V — for longer than the manufacturer-specified monitoring window. In a correctly functioning system, the downstream sensor (post-cat) should produce a relatively stable, low-oscillating signal around 0.45–0.55 V, confirming that the catalytic converter is efficiently scrubbing oxygen from the exhaust stream.
The most frequent root cause is a failed or contaminated oxygen sensor whose Nernst cell is internally shorted or saturated, causing it to report a rich-biased voltage regardless of actual exhaust composition. Wiring faults — particularly a short to a reference voltage line, damaged shielding, or corroded connector pins — can produce the same stuck-high signal. Because the downstream sensor on Sensor 2 is used for catalyst monitoring rather than closed-loop fuel trim, the engine will usually continue to run normally; however, closed-loop catalyst efficiency monitoring is compromised and emissions will typically exceed legal limits.
P0138 is closely related to P0137 (circuit low voltage, Bank 1 Sensor 2) and P0140 (no activity, Bank 1 Sensor 2). Always inspect the complete circuit before condemning the sensor, as a wiring short to voltage is a common misdiagnosis path that leads to unnecessary sensor replacement.
Common causes
Most-frequently reported root causes when P0138 is logged.
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1
Failed downstream oxygen sensor (Sensor 2, Bank 1) with internally shorted or saturated Nernst cell
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2
Short circuit in the sensor signal wire to a voltage reference or B+ line
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3
Corroded, damaged, or backed-out connector pins at the sensor harness
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4
Chafed or melted sensor wiring contacting an exhaust component
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5
Rich exhaust condition caused by fuel trim or injector fault causing a genuinely elevated sensor reading
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6
Engine coolant or oil contamination of the sensor element (silicone or phosphorus poisoning)
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7
Faulty ECM/PCM signal input circuit (rare; rule out sensor and wiring first)
Symptoms drivers notice
How to diagnose P0138
A typical diagnostic flow when this code is present.
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1
Perform a full scan for all stored and pending DTCs; note any co-codes (P0420, P013x, rich fuel trim codes) that may indicate a root-cause exhaust or fuelling fault
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2
Visually inspect the Bank 1 Sensor 2 wiring harness from the connector back to the ECM: look for chafing against exhaust pipes, melted insulation, corrosion at the connector, and backed-out terminals
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3
With ignition on, measure signal wire voltage at the sensor connector (disconnected from sensor): should read approximately 0.45 V reference bias; voltage above 1.0 V indicates a short to a voltage source in the harness
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4
With sensor connected and engine at operating temperature, observe live sensor voltage on a scan tool: a reading that is continuously stuck above 0.9 V and does not fluctuate confirms a failed sensor or a shorted circuit
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5
Measure sensor internal resistance between the signal and ground pins (should be within manufacturer spec, typically not open or shorted); also verify heater circuit resistance matches spec (usually 5–20 Ω)
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6
Check for exhaust leaks between the catalytic converter and Sensor 2 bung: a leak can dilute exhaust gases and skew readings, though more commonly causes P0137 (low voltage)
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7
Replace the oxygen sensor only after wiring and connector checks are clear; clear codes, complete a drive cycle, and confirm no reoccurrence
Related powertrain codes
- P0040 — Upstream Oxygen Sensors Swapped From Bank To Bank
- P0041 — Downstream Oxygen Sensors Swapped From Bank To Bank
- P0130 — O2 Sensor Circuit Malfunction (Bank 1 Sensor 1)
- P0131 — O2 Sensor Circuit Low Voltage (Bank 1 Sensor I)
- P0132 — O2 Sensor Circuit High Voltage (Bank 1 Sensor 1)
- P0133 — O2 Sensor Circuit Slow Response (Bank 1 Sensor 1)
Frequently asked questions
Can I drive with a P0138 code stored?
Yes, in most cases the vehicle will drive normally because the downstream Sensor 2 is not used for closed-loop fuel trim control. However, the catalytic converter efficiency monitor is disabled, so the vehicle will not pass an emissions (I/M) inspection, and a prolonged rich exhaust condition — if that is the root cause — can damage the catalyst over time.
Will P0138 clear itself after fixing the fault?
No. After repairing the underlying cause you must clear the DTC with a scan tool (or disconnect the battery, though that also resets other adaptations). The code will not self-erase; you then need to complete a full OBD-II drive cycle so the oxygen sensor and catalyst monitors run and confirm the repair.
How is P0138 different from P0137?
Both codes target Bank 1 Sensor 2, but P0137 indicates a persistently low voltage (below ~0.1 V — lean-biased or open circuit), while P0138 indicates a persistently high voltage (above ~0.9 V — rich-biased or short to voltage). Diagnosis starts from opposite ends of the circuit: P0137 points to open wiring or a lean exhaust leak, P0138 points to a wiring short to voltage or a saturated/failed sensor.
Is the oxygen sensor or the wiring more likely to be the problem?
On high-mileage vehicles (over 100,000 km / 60,000 miles) a failed oxygen sensor is the most common cause, as sensor lifespan is typically 5–8 years or 100,000–160,000 km. On lower-mileage vehicles, or when the code appears suddenly after exhaust work or a minor collision, inspect the wiring and connector first — a short to a reference voltage line is the most common wiring-side fault for this specific high-voltage code.
Disabling P0138 in software
RaceTune can permanently disable P0138 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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